A simple gas turbine cycle comprises a compressor, a combustion chamber located downstream of the compressor, and a turbine located downstream of the combustor. FIG. 3 (Prior Art) shows a conventional simple turbine cycle comprising a compressor 116, a turbine section 114, a combustion chamber 118, and a combustor 112. Compressed air from the compressor 116 is directed to the combustion chamber 118 and into the combustor 112 in which fuel, for example, natural gas, is burned in the presence of the compressed air. Hot gases exit the combustor 112 and enter into the turbine 114 where the gas expands to drive a rotor shaft 120. The shaft 120 is also responsible for driving the compressor 116 which consumes a portion of the power output. The remainder of the power output is utilized to drive a generator rotor (not shown in FIG. 3), thereby producing electricity.
Various components of gas turbine cycles have been used to enhance system efficiency and power output. For example, U.S. Pat. No. 5,465,569, entitled "Method of Establishing Part-Load Operation in a Gas Turbine Group," to Althaus describes a gas turbine system having a reheat cycle. The reheat cycle includes a low pressure combustor and self-igniting low pressure turbine disposed downstream of a simple turbine system like that shown in FIG. 3. U.S. Pat. No. 3,765,170, entitled "Composite Gas Turbine Set," to Nakamura describes a gas turbine system having a regenerative cycle. The Nakamura '170 Pat. includes a standard cycle (including a heat exchanger) ported to a second heat exchanger. This system includes a main turbine and auxiliary turbine which accepts bleed air from the main compressor and directs the gas from the auxiliary turbine outlet to the main turbine inlet. U.S. Pat. No. 2,755,621, entitled "Gas Turbine Installations with Output Turbine By-Pass Matching the Output Turbine Pressure Drop," to Terrell discloses a gas turbine system having a separate output turbine to accept the main turbine exhaust with by-pass control. U.S. Pat. No. 5,313,782, "Combined Gas/Steam Power Station Plant," to Frutschi et al. describes a system having dual compressors with dual, in-line turbines including a reheater and auxiliary intercooler. Each of the patents listed herein is incorporated by reference in its entirety.
FIG. 2 shows an enthalpy versus entropy (h vs. s) diagram of various cycles. Curve 1 of FIG. 2 represents a simple turbine cycle and Curve 2 of FIG. 2 represents a simple turbine cycle augmented by a reheat cycle. Beginning with ambient air, a compressor compresses the air from p1 to p4. The combustion process increases the enthalpy of the gas to h1 by increasing its temperature. A turbine expands the gas to a pressure p1 to complete the simple turbine cycle.
In the reheat cycle, illustrated by Curve 2, the gas is partially expanded to a pressure P2 through the High Pressure Turbine (HPT) component, which generally consists of one stage. A second combustor reheats the gas in order to increase the work capacity of the gas. The reheat temperature is generally assumed to be equal to the maximum Turbine Inlet Temperature (TIT), i.e., that which corresponds to h1. Higher TIT's mean more work output and higher cycle efficiency. Since the limiting factor on increasing TIT is the material used to construct the first stage of the turbine, research is underway to produce stronger and more heat-resistant materials and coatings. The reheat cycle presents an alternative to producing more power through higher and higher TIT.
The net power produced by the cycle can simply be measured by subtracting the length of the line of the compression path (4a-4b) from the length of the line of the expansion path (6a-6d). Curves 1 and 2 show that the reheat cycle has a longer expansion path line because it has two expansions (i.e., 6a-6b and 8a-8b). The efficiency of the cycle, however, is measured by dividing the power output by the energy of the fuel being used. A measure of the losses is the entropy generated within the cycle, which is represented by the distance from S1 to S4 for the reheat cycle.
As shown by Curve 2, the reheat process is capable of producing power at the same level as the simple cycle but at lower temperatures, which results in lower costs and longer lasting turbine blades and a more efficient cycle (i.e. that uses less fuel than a simple cycle). Further, the reheat process can produce more power than the conventional simple cycle at comparable efficiency levels using the same maximum TIT.
Although the reheat cycle provides some advantages over the simple cycle, the cost of equipment and fuel associated with operating a gas turbine system are high. Therefore, it is an object of the present invention to produce a gas turbine system having high efficiency, high power output for a given size of the components, and minimum overall equipment cost.